review: one fold implementation, uniform end-edge absence, partial text in tool errors, snapshot scenario

Address ds-review-bot on #2127:

- assistant-output: the rule has ONE implementation, the incremental
  AssistantOutputFold (push/pushText/collect); finalAssistantOutput folds a
  complete suffix, the SDK backend folds notification events, and the ACP
  backend folds raw chunk text into the same streamed fallback.
- subagent/end.lastAssistantMessage: 'no output' is encoded once — absent,
  never [], on both lifecycle shapes (observeRun now omits empty output).
- tool-subagent: a non-completed foreground result stays isError but appends
  the child's preserved partial text after the stop-reason headline.
- Authored keyless snapshot scenario subagent-max-tokens-partial pins the
  assembled transcript: the child's committed log carries the usage-only
  empty message and the parent's tool result carries the partial answer.
- Rule-boundary sentence (message wins over later streamed text) and the
  consumer half recorded in the Agent Note; comments trimmed to pointers.

packages/subagent/subagent-inprocess/src/index.ts

@@ -220,9 +220,7 @@ function readResult(
 ): SubagentResult {
   const own = child.session.events.slice(boundary)
   const lastEnd = findLastMessageTurnEnd(own)
-  // Canonical selection (`finalAssistantOutput`): the last non-empty assistant
-  // message, else the text streamed before cancel/error/truncation cut the
-  // turn short — an empty usage-only message never erases real output.
+  // The seam's canonical selection rule; a partial answer survives cancel and truncation.
   const output: ContentBlock[] = finalAssistantOutput(own) ?? []
   const recorded = toStopReason(lastEnd?.data.reason)
   // Disposal can tear the owner down before the loop records its ordinary